A transformer generally comprises primary and secondary insulated copper coils wound about a ferromagnetic core so as to be magnetically coupled thereto. The efficiency and performance of a transformer are limited by the electrical resistance of the coils and magnetization (hysteretic) losses of the core. Losses resulting from the coils and core are commonly referred to as "wire" and "core" losses. Wire losses result from the electric resistance of the coils of the transformers while core losses result from the magnetic hysteresis of the metallic laminations of the transformer core.
Wire losses and core losses either directly or indirectly generate heat which must be dissipated or the transformer will overheat and bum out. Generally, heat is dissipated through the structural surfaces of the transformer, i.e., through the transformer casing, or through radiators and cooling fins located thereon. As higher voltages and currents are transformed, more heat is generated by the wire and core losses. Larger electrical insulation distances and wire cross-sections require more space. Thus, for the transformation of larger powers, transformers increase in dimensions and weight, and require larger cooling surfaces.
The performance limitations of conventional transformers are overcome by superconducting transformers. Superconducting transformers experience lower losses than a conventional transformer and are thus more efficient and produce less heat. In addition, a superconducting transformer is more compact and lightweight than a conventional transformer. In general, a superconducting transformer comprises primary and secondary superconducting coils that are wound about a ferromagnetic core so as to be magnetically coupled thereto.
The superconducting coils are placed within a cryostat so as to maintain them at superconducting temperatures. The cryostat generally comprises a heat insulating tank that is filled with a cryogenic coolant so as to maintain the coils at superconducting temperatures. Although the tank is insulated, heat transfer to the inside of the tank can occur. A refrigeration unit is connected to the tank to dissipate this heat and cool the tank. As is well known, however, low temperatures adversely affect the efficiency of a refrigerator to remove heat. For example, in the low temperature environment that exists within the cryogenic tank, a penalty factor of about 10 to 25 is incurred. That is, about 10 to 25 watts of electrical input power will be required for the removal of 1 watt at 77 degrees kelvin.
The heat dissipation penalty factor increases the need to minimize all heat influx into the cryogenic tank so as to decrease the amount of input power that must be consumed by the refrigerator. For example, it has been attempted to lessen heat leakage occurring along the electrical connections by locating such connections internal to the tank so as to minimize hot-cold transitions. Even if the amount of heat generated is low, it is costly to dissipate in the low temperature environment of the cryogenic tank due to the penalty factor.
Core losses produce a significant amount of heat that must be dissipated. Due to the heat dissipation penalty factor, these core losses will offset performance efficiency gains from the superconducting coils. For this reason, cryostats for superconducting transformers have been provided with thermal insulation that isolates the core from the cryogenic coolant. For example, U.S. Pat. No. 5,107,240, issued to Tashiro ("the Tashiro patent"), discloses a superconducting transformer in which the core is isolated from the cryostat. The cryostat comprises an outer case having a first and second outer wall that are linked together by a central rod so that the outer walls and the central rod form the core of the transformer. The inner portion of the cryostat is isolated from the core by a fiber reinforced polymer double walled liner wherein a vacuum space is defined between the double walls. The polymer liner contains the cryogenic coolant into which the superconducting coils are submerged. The vacuum space between the double walls of the polymer liner insulates the core from the cryogenic coolant. In addition, the polymer liner prevents the formation of eddy currents in the walls of the cryostat tank.
The outer wall and the central rod of the cryostat described in the Tashiro patent comprise the core of the transformer. In addition to carrying the main flux of the transformer, the outer wall must also provide structural integrity for the cryostat, requiring that the outer wall be thicker. This provides more surface area for the formation of eddy currents and increases generated waste heat. In order to adequately insulate the core, the vacuum space between the walls of the inner liner must be enlarged. Expanding the vacuum space decreases the volume of the cryostat so that less cryogenic coolant can be contained therein which decreases the ability of the cryostat to maintain cryogenic temperatures.
The thermal insulation described in the Tashiro patent is not load bearing and is unable to withstand the pressure differential between the inside of the cryostat and the vacuum space. Thus, both the outer wall and the inner liner must be dimensioned so as to sufficiently support the pressure differential. The inner liner is also limited in that it is constructed from a polymeric material. The inner liner forms the cold interface of the cryostat. If not properly selected and designed to withstand such conditions, polymers can develop micro-cracks which can lead to cryogenic coolant, or heated gases, leaking into the vacuum space and destroying the thermal insulating property of the vacuum space between the hot and cold surfaces. In addition, polymers are gas permeable so that even if micro-cracks do not develop, the integrity of the vacuum space could be compromised by gas diffusion.
In the Tashiro patent, the electrical connections to the supply and load sides of the transformer are made external to the cryostat. Transformers are usually built as three-phase units, with electrical connections between phases for the formation of the desired vector group, and with internal connections between windings and a tap changer for the adjustment of the voltages. These connections should preferably be made inside the cryostat because each transition between cryogenic and ambient temperatures increases the parasitic heat leaks. In addition, internal connections can be made much more compact in the dense, electrically insulating cryogenic medium.
Currently, there is no cryostat that internally accommodates the windings and electrical connections for a three phase transformer while thermally insulating the core from the windings and is structurally strong. Thus, the need for such a cryostat exists.